Audi's S6 is a high-performance four-door sedan and the S7 is the automaker's high-performance five-door luxury hatchback. The two are platform-sharing siblings with common 420-horsepower twin-turbocharged 4.0-liter V8 engines, seven-speed dual-clutch automatic gearboxes and permanent all-wheel drive. No surprise, then, that straight-line performance between each is indistinguishable from one to the other. Based on this information, many casual observers are led to believe that the only discerni